The present invention relates to wireless networks, and more particularly to systems and methods for improving system timing.
Wireless bridge systems can efficiently and inexpensively interconnect data packet networks in campus areas by utilizing the unlicensed 2.4 GHz ISM band and 5 GHz UNIT bands. It is desirable to use popular wireless local area network (WLAN) standards such those belonging to the IEEE 802.11 family. In particular, the medium access control (MAC) procedures in the IEEE 802.11 standard can be used to coordinate and share the wireless medium between bridge stations. A problem arises in that the IEEE 802.11 MAC protocols assume that all wireless links have radio propagation delays of at most 1 microsecond. However, a point-to-multipoint wireless bridge system may have links that are miles long. Propagation delays on these links can be tens of microseconds, and can vary sharply among links.
The primary access protocol for 802.11 networks employs so-called “CSMA/CA” (Carrier-sense multiple access with collision avoidance) techniques. Contention-free periods are optionally provided within this CSMA/CA scheme. During contention periods, 802.11 stations can transmit if they believe the shared medium is free. The medium may be deemed to be not free based on either physical layer detection of a current transmission or based on a MAC layer detection of a current transmission. The MAC layer detection depends on monitoring of a duration field in received packets. The duration field may be found in the packet itself, or in a Request to Send (RTS) or Clear to Send message preceding the packet. When a new packet, RTS, or CTS is heard, a timer (the so-called NAV timer) can be set based on this duration field. Until this timer expires, the medium is considered to be busy. After a busy time is completed, each station will continue to defer transmission for a duration defined in part as a multiple of a locally computed random number and a system slot time.
When there is no shared understanding of system timing, there is a much higher probability of collisions due to breakdown of the MAC layer collision avoidance mechanism. Expiration of a prospective transmitter's NAV timer may not be a realistic indication of medium availability from the perspective of the intended receiver. At the conclusion of the busy period, a transmitter that begins a transmission right at the beginning of a slot may potentially collide with other transmitters that are beginning transmission on that slot or some portion of the previous slot due to the varying understanding of the slot boundaries and delayed detection of simultaneous transmissions due to link propagation delays. As link distances increase, packet collision probabilities will also increase unless timing boundaries are well understood at all stations.
Furthermore, physical layer carrier sense mechanisms may not be helpful in a wireless campus network due to the well-known hidden terminal problem. A root bridge of the wireless campus network typically uses an omni-directional antenna while the non-root bridges use directional antennas pointed at the root bridge. Thus the root bridge may hear multiple simultaneous transmissions from non-root bridges that collide because they do not hear each other. Thus, much larger collision probabilities can be expected when 802.11 techniques are extended to campus-scale wireless networks.
FIG. 1A depicts a simple example indicative of the type of problems that may arise. A wireless network includes a root bridge (RB) and two non-root bridges (NRB1 and NRB2).    1) NRB1 starts to transmit a frame, t1 before the timing boundary of RB (t0). NRB1 and NRB2 cannot hear each other.    2) RB consider the wireless medium “idle” if t1 is less than the propagation delay between RB and NRB1 (T1). Then RB starts to send a frame at t0.    3) NRB2 consider the wireless medium “idle” if t2 is less than the propagation delay (T2) between RB and NRB2. Then NRB2 starts to send a frame at to+t2.    4) The packet collision duration for the 1st slot of RB is therefore expanded to t1+Tslot+t2.    5) The maximum packet collision duration for the 1st slot of RB is T1+Tslot+T2. Thus average packet collision probability also increases in proportion to 1+(T1+T2)/(2*Tslot).
A more complete quantitative analysis of this effect will be presented later in this document. What is needed are systems and methods for adapting 802.11 techniques to networks with larger propagation delays such as campus point-to-multipoint wireless networks.